Shortly before 3 p.m., Twitter sent a suspension of service notice to De-De, the design firm behind Thunderclap. "This is a notice that your [authorization] token for Thunderclap has been suspended from interacting with the Twitter API," the message read. "Please make sure that your application follows Twitter's API Terms of Service."

I've left messages with Twitter's PR team asking what the specific violation of the terms of service was. I'll update if and when I find out.

At that scale, Twitter could well view Thunderclap as a potential competitor to its Promoted Tweets ad product. Thanks to the success of Promoted Tweets, Promoted Trends and Promoted Accounts, Twitter is already generating more revenues from its mobile business than from its desktop site, CEO Dick Costolo revealed this week. That stands in stark contrast to Facebook, which has had serious trouble monetizing its mobile audience.

Update: These appear to be the relevant portions of the Twitter terms of service.

Spam: You may not use the Twitter service for the purpose of spamming anyone. What constitutes “spamming” will evolve as we respond to new tricks and tactics by spammers. Some of the factors that we take into account when determining what conduct is considered to be spamming are:

If you post duplicate content over multiple accounts or multiple duplicate updates on one account;

[From the section on automation rules and best practices:] The @reply and Mention functions are intended to make communication between users easier, and automating these processes in order to reach many users is considered an abuse of the feature. If you are automatically sending @reply messages or Mentions to many users, the recipients must request or approve this action in advance. For example, sending automated @replies based on keyword searches is not permitted.

Thunderclap works by allowing Twitter users to authorize the service to retweet a message from their accounts at a given time, producing a simultaneous burst of tweets. In essence, Twitter seems to be taking the view that Thunderclap, as a single entity thousands of @replies at a time -- albeit on behalf of users who gave their consent -- it's practicing a prohibited form of automation, equivalent to spamming.